Research On Calculation Model Of Urban Road Network Traffic Carrying Capacity Based On Equivalent Traffic Flow

The traffic carrying capacity of urban road network reflects the road network’s ability to deal with traffic demand,and is the threshold for the sustainable development of urban transportation.It has important theoretical and practical significance for alleviating the imbalance of traffic supply and demand,diagnosing the crux of urban traffic problems,and formulating urban control planning.Existing related works have proposed calculation models and solutions for different application environments.However,these studies have problems such as diversification of capacity definitions,incorrect analysis of factors,excessive model constraints,and difficulty in determining parameters.This brings great challenges to the integrated development,coupling coordination and cross application of road network traffic carrying capacity.In response to these problems,it is urgent to deeply analyze the characteristics and attributes of existing concepts and calculation models,clear the essential connotation of traffic carrying capacity,clarify the key influencing factors of traffic carrying capacity,build a calculation model for traffic carrying capacity,and achieve effective application of traffic carrying capacity.The research results have an important role in enriching the theory of optimal control of traffic supply-demand,improving urban traffic congestion issues,and promoting the sustainable development of urban traffic.The existing basic theory of road network traffic carrying capacity needs to be improved and the recognized method system has not been formed.The definition of traffic carrying capacity has not been unified.Most of the related concepts are confined to static time and space resources,which makes it unable to effectively serve the dynamic practical needs and difficult to carry out systematic research.Therefore,this paper analyzes the essential attributes of road network traffic carrying capacity and clarifies its connotation,that is,the maximum number of vehicles travelling simultaneously in the road network under a certain service level.Based on the consideration of the variability of traffic demand and the dynamics of time-space resources,this paper aims to establish a dynamic interactive mapping correlation between road network service level and road network traffic carrying capacity,realize the quantitative characterization and efficient calculation of road network traffic carrying capacity,and construct a calculation model of road network traffic carrying capacity that is both theoretical and practical.During this process,this paper focuses on the basic attributes of traffic carrying capacity,and carries out research from three aspects:analysis of influencing factors,construction of calculation model and design of application scenarios.The analysis of influencing factors is the theoretical basis of research,the construction of calculation model is the key point of research,and the design of application scenarios is the expansion practice of research.The main research contents are as follows:(1)Analysis of the essential characteristics and key influencing factors of road network traffic carrying capacity.Firstly,this paper evaluates the relevant concepts of traffic carrying capacity,displays the basic characteristics and main drawbacks of different traffic carrying capacity calculation models,and expounds the necessity of proposing the concept of traffic carrying capacity.Then,the characteristic attributes of traffic carrying capacity are clarified,including inherent attributes of road network,time dimension independence and service level mapping.Finally,through the deconstruction and induction of different influencing factors of traffic carrying capacity,the key parameters of influencing factors are determined from the macro and micro levels,the causal correlation and logical structure between the intrinsic factors are analyzed,and the key influencing factors of urban traffic carrying capacity are determined as road network structure,signal timing and service level.(2)Construction of calculation model of basic unit traffic carrying capacity based on equivalent traffic flow.The analysis of road network structure is the basis of traffic carrying capacity modeling.By exploring the minimum repetitive structure of road network,the basic unit of road network traffic is determined.According to the results of road network division,the modeling process of road network traffic carrying capacity can be transformed into two steps: calculation of basic unit traffic carrying capacity,relationship between traffic carrying capacity of basic unit and traffic carrying capacity of road network.Basic unit traffic carrying capacity modeling is the core and foundation of road network traffic carrying capacity modeling.Aiming at the construction of calculation model of basic unit traffic carrying capacity,firstly,through the analysis of the operating mechanism of basic unit traffic flow,it is found that the traffic flow shows periodic stop-go behavior,which significantly increases the complexity of the modeling.In order to solve this problem,the concept of equivalent lane is put forward.Based on the principle of unchanged average travel speed,it converts the interrupted traffic flow controlled by signals into continuous release traffic flow on the premise of ensuring the same number of vehicles in the road network.After equivalent conversion,some lanes at the intersection may be blocked.In order to calculate the traffic carrying capacity including the blocked flow,the idea of traffic flow abstraction is designed.According to the law of travel time conservation,the traffic flow is divided into two alternative modes: free travel and stacked release.The calculation models of travel time in stacked release zone and free travel zone are established respectively.Finally,based on the converted equivalent flow,considering the spatiotemporal interaction and mapping correlation between the traffic flows in different road sections,the traffic carrying capacity of various sections such as the stacked release zone and the free travel zone is determined,and then the calculation model of basic unit traffic carrying capacity based on equivalent traffic flow is constructed.(3)Establishment of calculation model of road network traffic carrying capacity considering the difference of traffic conditions.Based on the calculation model of basic unit traffic carrying capacity,this part further discusses the correlation pattern and interaction mode between traffic carrying capacity of basic unit and traffic carrying capacity of road network.To this end,the operating characteristics of road network traffic flow are analyzed firstly,and it is found that the overall traffic condition of road network is not stable and uniform,and the average travel speed among basic units may be different,which affects the establishment of direct mapping correlation between average travel speed and traffic carrying capacity of road network.In view of this,at the beginning of the modeling,the influence mechanism of homogeneous and non-homogeneous conditions on road network traffic carrying capacity is analyzed.Through multi-dimensional logical derivation,mathematical demonstration and scenario deduction,it is proved that the difference of traffic condition among basic units under different structure levels does not affect the numerical value of road network traffic carrying capacity.The traffic carrying capacity of road network is always equal to the sum of traffic carrying capacity of basic units based on the average travel speed of road network.Then the calculation model of road network traffic carrying capacity is constructed.After the model is built,the correctness and effectiveness of the model are verified based on the multi-structure simulation road network.Through sensitivity analysis,the mechanism of dynamic and static parameters on road network traffic carrying capacity and the improvement of road network service level are analyzed.The characteristics and applicable scenarios of vehicle accumulation in the MFD,narrow road network capacity,generalized road network capacity and road network traffic carrying capacity are quantitatively compared.The results show that the four models describe the network performance from the threshold,throughput,accommodation and operation of traffic flow respectively.The research can provide theoretical basis and method support for traffic managers to choose suitable road network performance evaluation model.(4)Proposal of urban traffic intelligent control strategy based on road network traffic carrying capacity.The calculation model of road network traffic carrying capacity can provide data support and application guidance for different aspects of urban traffic control.Taking road network perimeter coordination control and urban car ownership control as the examples,this paper introduces the social value and guiding role of road network traffic carrying capacity,and further verifies the practicability and reliability of the road network traffic carrying capacity calculation model from the application level.In the aspect of road network perimeter coordination control,a new road network perimeter control framework composed of data collector,sub-region coordination controller and perimeter signal controller is developed.Taking the traffic carrying capacity as the critical threshold,the vehicle distribution mode is optimized through the traffic flow transfer of sub-regions,and the traffic efficiency of road network is effectively improved.In terms of urban car ownership control,based on the mathematical correlation that "car ownership is the ratio of car traveling volume to car operation rate",the car traveling volume estimation method based on traffic carrying capacity is proposed,and the car operation rate prediction model based on combination model of hunter-prey optimizer algorithm and bi-directional long short-term memory neural network is constructed.Then,the control models of car ownership under different service levels are determined,and the upper limit of control scale and dynamic optimization opinions are given.To sum up,this paper takes the characteristic attributes of road network traffic carrying capacity as the pointcut,focuses on the key influencing factors of road network traffic carrying capacity,takes average travel speed as the characteristic variable of service level,and aims to establish the interactive correlation among average travel speed,road network structure parameters,signal timing parameters and other parameters,thereby determining the dynamic mapping relationship between road network service level and road network traffic carrying capacity.During this process,the proposal of equivalent lane is to realize the conversion from intermittent flow to continuous flow;the abstraction of traffic flow divides flow operation into alternative modes of free travel and stacked release;the exploration of basic unit refines the structure of road network and realizes the effective simplification of road network traffic carrying capacity modeling process.The research results not only improve the calculation method of urban traffic carrying capacity,realize the quantitative characterization of road network carrying performance,enrich the theory system of traffic supply-demand structure optimization control,but also provide theoretical guidance and technical support for the scale control of car ownership,coordinated control of network perimeter traffic,optimal allocation of road space-time resources,etc.